Adding thickness to 3D Scan of face in Blender
The 3D print plugin is nice for fixing up a mesh but otherwise I used to just use the decimate modifier and create a new mesh from a scan mesh. Also make sure you've tried selecting all vertices in edit mode and using the 'remove double vertices' function (or remove doubles in 2.79). Then using Crtl + Shift + N to set the normals (untick that box in the bottom left corner though) (in 2.79 it's just Crtl + N and there's no tickbox).

What it looks like is that for whatever reason the scan mesh isn't whole, like there's separate islands of vertices which would give the effect of the cracks